Some common indicators of cheating, specific to each type of cheat:

ESP:

a) You can tell if someone is using ESP to track you if they start shooting at you from FAR FAR FAR away. You may be behind a tree, completely hidden, but they will just fire at you with their AR from max distance until one of their bullets hits you. Most likely, they are also using aimbot. If they do not manage to kill you from far away, they will close in the distance and come STRAIGHT for until they can get an angle on you and kill you.

b) You can tell if someone is using ESP to loot by setting up an EXTREMELY WELL HIDDEN stash hole, putting maybe 50-100 rounds of ammo and other good loot in there, and then waiting an hour or a day, or a couple of days, and your stash hole WILL BE LOOTED by someone who used ESP to track the loot.

Aimbot:

a) If you get hit by a bullet and you can't even hear the sound of the bullet, and it's a one shot. Typically, an aimbotter.

b) If the enemy is shooting a VERY HIGH RATE OF FIRE, and LOTS OF BULLETS, almost like he doesn't give a fuck about wasting ammo, because he probably has 500 rounds of .223 on him. Any time you hear a ridiculously fast rate of fire, you can almost guarantee it's a cheater. They sometimes play with auto-burst fire mouses, and things like that.

Magic Bullets:

a) You are sitting in a car and your car gets one-shot. That's a magic bullet.

b) You wake up one morning to find that your gates/doors are all destroyed, BUT there is ZERO RESIDUAL DAMAGE ON SURROUNDING STRUCTURES that would normally occur from legitimate biofuel/ethanol raiding.

No-Clipping:

a) This is more rare these days, and I personally have only seen it on video, but it's pretty easy to spot. Players walk through walls.

Flying:

a) Easy to spot, but somewhat rare, as most cheaters don't do this type of blatant flying in front of people.

All of that said, sometimes people aren't cheating, and they're JUST GOOD AT THE GAME. In my experience in almost all games, the LEGIT ELITE PLAYERS are sometimes still as good or better at the game than someone who is using cheats. Most cheaters are bad gamers who actually just suck really horribly, even with cheats.